It is known to use electron beams and light beams to excite a target screen as one step in developing a viewable picture, either monochrome or color. For color rendition, by far the most popular form of display device is the so-called "shadow-mask" cathode ray tube. It is also known, however, that there is another more efficient way to control the generation of a color picture; by synchronizing the excitation of the target screen via index signals which provide information as to the location of the electron beam or light beam as set forth for example in U.S. Pat. No. 3,967,315.
Another category of display devices exists which develops high resolution viewable images in color, the so-called penetration cathode ray tube. The voltage at the target screen in these penetration tubes is switched over the range from 6-18 kilovolts to change the color of the image being generated.
Also, very high resolution kinescopes in monochrome are used to present alpha-numeric or graphic type data. The performance of these kinescopes is reaching the stage where the quality of the image developed approaches that of the printed page with four point type.
In all the above types of display devices there are problems having to do with the power supplies required for their operation. For example, to operate the "shadow-mask" tube a high voltage supply typically furnishes 25-30 kilovolts with an average current of approximately 1 milliampere. Two serious problems are associated with power supplies capable of providing such power. First, there is the matter of health and safety for the operator or the maintenance technician. The voltages and power involved are lethal. Second, there is the matter of display system malfunction which can be brought about by high voltage arcing between elements of the kinescope. Various forms of arc suppression are in use but each has its limitations. Furthermore, some sensitive circuits can be destroyed even when high voltage arcs are shunted to ground to protect the tube.
In the penetration type kinescope, it is possible to achieve very high resolution in any single color because of the homogenous construction of the different color producing phosphor layers. However, to achieve correct registration of the image from one color to the next presents a serious problem because the deflection sensitivity of the display system has to track large changes in high voltage to a very high degree of precision.
In the high resolution monochrome kinescope a similar registration problem arises because as the resolution gets better and better the requirement to control the exact position of the electron beam continues to increase.
In the beam-index color tube, the problem is the least severe but the stability and energy storage of the power supplies still are matters of concern.
For the above reasons, the cost and energy utilization of power supplies in modern high resolution and color display devices are becoming leading factors in the overall performance and selling price of the complete display system.
Additionally, in terms of energy-efficiency, the penetration type color tube suffers from the usual practice of `killing` one of the phosphors in order that it not respond to low levels of excitation. Nevertheless, despite this practice (which results in lower screen brightness) and the requirement for fast switching of the high voltage, and the limited range of colors that are provided, the penetration tube is often selected today as the display medium because it yields high resolution in the vector (stroke) writing mode.